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Tag: Cheerios effect

  • Active Cheerios Self-Propel

    Active Cheerios Self-Propel

    The interface where air and water meet is a special world of surface-tension-mediated interactions. Cereal lovers are well-aware of the Cheerios effect, where lightweight O’s tend to attract one another, courtesy of their matching menisci. And those who have played with soap boats know that a gradient in surface tension causes flow. Today’s pre-print study combines these two effects to create self-propelling particle assemblies.

    The team 3D-printed particles that are a couple centimeters across and resemble a cone stuck atop a hockey puck. The lower disk area is hollow, trapping air to make the particle buoyant. The cone serves as a fuel tank, which the researchers filled with ethanol (and, in some cases, some fluorescent dye to visualize the flow). Like soap, ethanol’s lower surface tension disrupts the water’s interface and triggers a flow that pulls the particle toward areas with higher surface tension. But, unlike soap, ethanol evaporates, effectively restoring the interface’s higher surface tension over time.

    With multiple self-propelling particles on the interface, the researchers observed a rich series of interactions. Without their fuel, the Cheerios effect attracted particles to each other. But with ethanol slowly leaking out their sides, the particles repelled each other. As the ethanol ran out and evaporated, the particles would again attract. By tweaking the number and position of fuel outlets on a particle, the researchers found they could tune the particles’ attractions and motility. In addition to helping robots move and organize, their findings also make for a fun educational project. There’s a lot of room for students to play with different 3D-printed designs and fuel concentrations to make their own self-propelled particles. (Research and image credit: J. Wilt et al.; via Ars Technica)

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  • Enhancing the Cheerios Effect

    Enhancing the Cheerios Effect

    The Cheerios in your morning cereal clump together with one another and the bowl’s wall due to an attractive force caused by the curvature of their menisci. A recent study looks at how this effect changes when you’re pulling objects out of the liquid.

    Snapshots show how two flexible fibers get drawn together by an attractive force as they are pulled out of silicon oil.
    Snapshots show how two flexible fibers get drawn together by an attractive force as they are pulled out of silicon oil.

    The researchers inserted thin flexible glass fibers into silicon oil and withdrew them. As they did, they explored what lengths and retraction speeds caused the fibers to pull together. They found that a single moving rod had a taller meniscus than a stationary one, and two moving rods had a liquid bridge that superposed their individual menisci. The result was an attractive force even stronger than what the fibers experienced when still. (Image credit: Cheerios – D. Streit, experiment – H. Bense et al.; research credit: H. Bense et al.; via APS Physics)

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    Mermaid Cereal

    In the Cheerios effect, floating objects can fall into one another due to capillary attraction — just like Cheerios link up in a cereal bowl. Here researchers play with that effect by adding repulsive magnets to their “cereal” pieces. They find that their so-called mermaid cereal falls into preferential spacing, with pieces pairing up but never touching. Adding lots of these pieces in a confined space creates interesting crystalline and striped patterns, as seen later in the video. (Video credit: A. Hooshanginejad et al.)

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    A Fractal Raft From a Spinning Top

    File this one under Cool Things I Would Have Never Thought Of. In this video, researchers play around with the flow around a spinning top and end up creating a fractal, granular raft. By immersing a top in dyed fluid, they show the toroidal vortices that form around the spinning toy. Then, instead of dye, they add a stretchy elastomer compound that cures over time. The elastomer stretches into thin ligaments in the swirling flow around the top. Eventually, it breaks apart into spherical drops of all different sizes.

    Once the top is removed, the elastomer drops slowly float to the surface. Surface tension and the Cheerios effect draw the drops together, and because of their many sizes, the rafts that form are fractal. (Image and video credit: B. Keshavarz and M. Geri)

  • Artificial Microswimmers

    Artificial Microswimmers

    Tiny organisms swim through a world much more viscous than ours. To do so, they swim asymmetrically, often using wave-like motions of tiny, hair-like cilia along their bodies. Mimicking this behavior in artificial swimmers is tough; how would you actuate so many micro-appendages? A new study offers a different method: inducing cilia-like waves using magnetic fields.

    The researchers’ microswimmers are actually arrays of ferromagnetic particles. The Cheerios effect helps draw the particles together, while magnetic repulsion pushes them apart. Together, these forces help the particles assemble into crystal-like arrays.

    To make the particles swim, the researchers shift the magnetic field. All of the outer particles of the array behave like individual cilia. As the magnetic field moves, the cilia-particles move in waves, much like their natural counterparts. Using this technique, the researchers were able to demonstrate both rotational and straight-line (translational) swimming. (Image, research, and submission credit: Y. Collard et al.)

  • Breaking Up Granular Rafts

    Breaking Up Granular Rafts

    Particles at a fluid interface will often gather into a collection known as a granular raft. The geometry of the interface where it meets individual particles, combined with the surface tension, creates the capillary forces that attract these particles to one another. Colloquially, this is called the Cheerio’s effect; it’s the same physics that draws those cereal chunks together in your bowl.

    Once together, these granular rafts can be surprisingly difficult to break up. That’s the focus of a new study on erosion in granular rafts. As seen in the top image, the raft has to be moving quite quickly before individual beads get pulled away. The experimental set-up here is pretty neat, and it’s not apparent from the video, so I’ll take a moment to explain it. The particles you see are gathered at an interface between water and oil. To generate the movement we see, researchers take the metal cylinder seen at the left of the image and pull it downward. That curves the oil-water interface, effectively creating a hill for the raft to accelerate down.

    To focus in on the forces necessary to separate individual particles, the researchers also looked at a pair of particles (bottom image). With this set-up, they could more easily track the geometry of the contact line where the oil, water, and bead meet. What they found is that the attractive forces generated between the beads are two orders of magnitude larger than predicted by classical theory. To correctly capture the effect, they needed a far more precise description of the contact line geometry around a sphere than is typically used. (Image and research credit: A. Lagarde and S. Protière)

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    “Focus, Vol. 1”

    In “Focus, Vol. 1,” photographer Roman De Giuli follows colorful droplets as they roll along, chase one another, and burst. You may notice that many of the drops seem attracted to one another. This is actually a surface tension effect caused by the dimples the droplets create on the surface; it’s the same effect responsible for Cheerios clumping together in your milk. Interestingly, though, the oil coating the drops doesn’t seem to drain quickly enough for the clumping drops to actually coalesce. (Image and video credit: R. De Giuli)

  • Collective Motion: Nematodes

    Collective Motion: Nematodes

    We often imagine that collective motion creates an advantage – that the schooling fish and flocks of birds gain something from this behavior – but that’s not always the case. Above, you see nematodes moving through a thin liquid layer. Random collisions occasionally bring the nematodes into contact, and once that happens, surface tension holds them together with a force that exceeds what their muscles can supply. Essentially, they move together for the same reason that Cheerios clump together in your cereal bowl. But despite being stuck alongside one another, there’s no change in how the nematode moves. It sees neither an advantage nor a disadvantage from being attached to its neighbor. (Image and research credit: S. Gart et al., source)

    This post completes our series on collective motion. Check out the previous posts about honeybee waves, how crowds are like sand, the fluid properties of worms, and why a lack of randomness makes predicting group behaviors hard.

     

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    The Cheerios Effect

    You’ve probably noticed that cereal clumps together in your breakfast bowl, but you may not have given much thought as to why. This tendency for objects at an interface to attract is known as the Cheerios effect, although it happens in more than just cereal, as Joe Hanson from It’s Okay to Be Smart explains. The effect is a combination of buoyancy, gravity, and surface tension acting in concert.

    When air, a liquid, and a solid meet, they form a meniscus, the curvature of which depends on characteristics of their interaction. Light, buoyant cereal and the walls of your bowl both have upward-curving menisci. Denser objects, like the tacks shown below, stay at the surface only because surface tension holds them up. Their meniscus curves downward.

    Objects with a similar meniscus curvature will attract. For cereal approaching a wall, the light Cheerio is buoyant enough that there’s an upward force on it, but it’s constrained to stay at the interface. It cannot rise, but that buoyancy is enough to let it climb the meniscus at the wall. The two tacks attract one another for similar reasons, except this time their weight helps them fall into one another. Check out the full video to see more examples of this effect in nature! (Video and image credit: It’s Okay to Be Smart; research credit: D. Vella and L. Mahadevan, pdf)

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  • A Particle-Filled Splash

    A Particle-Filled Splash

    A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s – it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video.  (Image credit and submission: A. Sauret et al., source)